This invention relates to a moisture sensitive device that measures relative humidity using the proportional relationship between relative humidity and electrical resistivity. More particularly, the invention relates to a new moisture sensitive device that is useful, e.g., to indicate when cooking has been completed in a microcomputer-controlled microwave oven by measuring the the humidity in the oven, or to control the amount of steam supplied by a humidifier in an FF (forced flue) heater.
The recent advance of electronic control technology has been remarkable, and many home appliances, such as microwave ovens, FF heaters, and air conditioners are now operated under the control of a microcomputer. Of course it would be desirable that if a microcomputer is used at all, it should be able to make decisions and carry out sophisticated job functions other than just telling time (i.e., a timer's function) and executing a prestored program, but to do this, various types of information must be gathered and supplied to the microcomputer and, particularly, a moisture sensitive device is necessary for gathering information on humidity.
Moisture sensitive devices are known that measure relative humidity by converting relative humidity into electrical resistivity which is proportional to relative humidity. A conventional moisture sensitive device of this type uses a metal oxide (e.g., Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, or NiO) as a moisture sensitive element. However, a moisture sensitive device using such a metal oxide does not have a relative humidity versus electrical resistivity profile that satisfactorily meets all requirements for practical use. For instance, a moisture device using NiO is not very sensitive at a relative humidity lower than 60%, and those using Fe.sub.2 O.sub.3 or Al.sub.2 O.sub.3 have an undesirably high resistivity for some values of relative humidity. A moisture sensitive device using a material that consists mainly of lithium chloride is also well known, but since it is deliquescent, the salt may become liquid under high humidity conditions, rendering the device inoperable.
Therefore improved moisture sensitive devices have been desired.